This project is based on the concept that a cause of hypertension may be lowered permeability in the capillaries, with age, requiring higher arterial pressure to maintain transvascular fluid movement (TVM). The method of measuring this fluid movement is based on swelling of the rodent tail under a slight vacuum. It has been shown by the author that this TVM decreases with age. This study will be conducted in SHR strain of rats (genetically hypertensive) and compared with non-hypertensive WKY rats as controls; measurements will start with weanlings and continue until adulthood.